1. Field of the Invention
The present invention relates to an automatic transmission exclusively for an automobile used with a torque converter, particularly to a structure of an automatic transmission mechanism.
2. Description of the Prior Art
An automatic transmission, in general, comprises two single planetary gear units, a sun gear which is commonly used with said two single planetary gear units, a carrier of the first planetary gear unit and a ring gear of the second planetary gear unit which are coupled together. The above structure is a so-called Simpson type wherein driving force is input to a ring gear of the first planetary gear unit and/or sun gear selectively, rotary components of the planetary gear units are stopped by a brake or one-way clutch, and output is taken from a ring gear of the second planetary gear unit which is coupled with a carrier of the first planetary gear unit. Then, forward three speeds are obtained.
A four speed automatic transmission is structured based on said forward three speed automatic transmission mechanism comprising two single planetary gear units. In addition to said three speed automatic transmission mechanism, by attaching an over-drive mechanism or under-drive mechanism having one planetary gear unit, forward four speeds are obtained.
An automatic transmission mechanism having an over-drive mechanism is axially arranged because an output member of the over-drive, i.e, a ring gear is coupled axially by an input member of the three speed automatic transmission.
On the other hand, said automatic transmission mechanism having an over-drive mechanism has a counter drive gear at the end of an output shaft (farthest end from an input portion) from which torque is transmitted to an under-drive mechanism placed under the three speed automatic transmission mechanism.
Recently, an automatic transmission has been required to minimize its body based on an increase of a power of an engine and tendency of "Front Engine and Front Drive" style. However, said four speed automatic transmission having three planetary gear units with an O/D mechanism can not satisfy such requirements.
Then, another four speed automatic transmission adding two clutches on a planetary gear unit of Simpson type is proposed. The structure of said automatic transmission is that a third clutch is laid between a sun gear of first planetary gear unit and a sun gear of a second planetary gear unit, (said sun gears are not separated in Simposn type) and the sun gear of the first gear unit is coupled with a carrier of the second gear unit through a fourth clutch.
The operation of the above structure is that the first gear unit and second gear are disconnected by the third clutch, driving force is transmitted to a carrier of the second gear unit by connecting fourth clutch, the sun gear of said second gear unit is fixed, and the overdrive is taken from the ring gear, and then, fourth speed is obtained in addition to third speed.
The above mentioned improved four speed automatic transmission can not satisfy requirement to minimize its body because the structure, like three speed automatic transmission of Simpson type, is axially long, i.e., driving force is input to one end and taken from the other end of the mechanism and both gear units are separated.
Furthermore, as an output member is laid at the end of the shaft where replacement and modification can be done easily, clutches can not be laid. Accordingly, three speed and four speed automatic transmissions have to be produced separately. Due to the above, it is difficult to produce parts and assemble them in common for said two kinds of transmissin mechanisms, so, cost increase is inevitable, and such structure of an automatic transmission can not cope with wide variation of car types.